Julianne I. Moses

DISEQUILIBRIUM CARBON, OXYGEN, AND NITROGEN CHEMISTRY IN THE ATMOSPHERES OF HD 189733b AND HD 209458b

We have developed a one-dimensional photochemical and thermochemical kinetics and diffusion model to study the effects of disequilibrium chemistry on the atmospheric composition of “hot-Jupiter” exoplanets. Here we investigate the coupled chemistry of neutral carbon, hydrogen, oxygen, and nitrogen species on HD 189733b and HD 209458b and we compare the model results with existing transit and eclipse observations. We find that the vertical profiles of molecular constituents are significantly affected by transport-induced quenching and photochemistry, particularly on the cooler HD 189733b; however, the warmer stratospheric temperatures on HD 209458b help maintain thermochemical equilibrium and reduce the effects of disequilibrium chemistry. For both planets, the methane and ammonia mole fractions are found to be enhanced over their equilibrium values at pressures of a few bar to less than an mbar due to transport-induced quenching, but CH4 and NH3 are photochemically removed at higher altitudes. Disequilibrium chemistry also enhances atomic species, unsaturated hydrocarbons (particularly C2H2), some nitriles (particularly HCN), and radicals like OH, CH3, and NH2. In contrast, CO, H2O, N2, and CO2 more closely follow their equilibrium profiles, except at pressures 1 μbar, where CO, H2O, and N2 are photochemically destroyed and CO2 is produced before its eventual high-altitude destruction. The enhanced abundances of CH4, NH3, and HCN are expected to affect the spectral signatures and thermal profiles of HD 189733b and other relatively cool, transiting exoplanets. We examine the sensitivity of our results to the assumed temperature structure and eddy diffusion coefficients and discuss further observational consequences of these models.

CHEMICAL CONSEQUENCES OF THE C/O RATIO ON HOT JUPITERS: EXAMPLES FROM WASP-12b, CoRoT-2b, XO-1b, AND HD 189733b

Motivated by recent spectroscopic evidence for carbon-rich atmospheres on some transiting exo-planets, we investigate the influence of the C/O ratio on the chemistry, composition, and spectra of extrasolar giant planets both from a thermochemical-equilibrium perspective and from consideration of disequilibrium processes like photochemistry and transport-induced quenching. We find that although CO is predicted to be a major atmospheric constituent on hot Jupiters for all C/O ratios, other oxygen-bearing molecules like H2O and CO2 are much more abundant when C/O < 1, whereas CH4, HCN, and C2H2 gain significantly in abundance when C/O > 1. Other notable species like N2 and NH3 that do not contain carbon or oxygen are relatively unaffected by the C/O ratio. Disequilibrium processes tend to enhance the abundance of CH4, NH3, HCN, and C2H2 over a wide range of C/O ratios. We compare the results of our models with secondary-eclipse photometric data from the Spitzer Space Telescope and conclude that (1) disequilibrium models with C/O ~ 1 are consistent with spectra of WASP-12b, XO-1b, and CoRoT-2b, confirming the possible carbon-rich nature of these planets, (2) spectra from HD 189733b are consistent with C/O ≲ 1, but as the assumed metallicity is increased above solar, the required C/O ratio must increase toward 1 to prevent too much H2O absorption, (3) species like HCN can have a significant influence on spectral behavior in the 3.6 and 8.0 μm Spitzer channels, potentially providing even more opacity than CH4 when C/O > 1, and (4) the very high CO2 abundance inferred for HD 189733b from near-infrared observations cannot be explained through equilibrium or disequilibrium chemistry in a hydrogen-dominated atmosphere. We discuss possible formation mechanisms for carbon-rich hot Jupiters, including scenarios in which the accretion of CO-rich, H2O-poor gas dominates the atmospheric envelope, and scenarios in which the planets accrete carbon-rich solids while migrating through disk regions inward of the snow line. The C/O ratio and bulk atmospheric metallicity provide important clues regarding the formation and evolution of the giant planets.

The effects of external material on the chemistry and structure of Saturn's ionosphere

We have developed a one-dimensional coupled ion-neutral photochemical model for Saturns upper atmosphere to better understand the structure and chemistry of Saturns ionosphere. In addition to modeling the chemistry of hydrogen and hydrocarbon ions, we investigate the effects of an oxygen and metal influx from ring or meteoric sources. The Infrared Space Observatory observations of H2O and CO2 in Saturns stratosphere are used to constrain the influx of extraplanetary material. As expected, the topside ionosphere of Saturn is dominated by H+, with H3+ prevailing just below the electron density peak. When micrometeoroid ablation is considered, we find that metal ions, represented here by Mg+, can take the place of hydrocarbon ions as the major ionic species in the lower ionosphere. The models then exhibit a characteristic double peak, with H+ creating the high-altitude peak and Mg+ the low-altitude peak. A pronounced gap forms between the two peaks, especially at night, when H3+ ions rapidly recombine. Neutral winds and electric fields in the presence of magnetic fields can cause vertical plasma motion that can shift the location of both electron density peaks. In addition, multiple sharp layers in the electron density profile can form in the lower ionosphere when oscillatory vertical drifts are introduced into the model to simulate the effects of atmospheric gravity waves. The location and magnitude of the “main peak” as well as the sharper lower-ionospheric layers observed with the Voyager and Pioneer radio occultation experiments (and eventually with similar Cassini observations) can help constrain the atmospheric structure, wind profiles, or electric field properties in Saturns upper atmosphere.

Hydrocarbon nucleation and aerosol formation in Neptune's atmosphere

Photodissociation of methane at high altitude levels in Neptunes atmosphere leads to the production of complex hydrocarbon species such as acetylene (C2H2), ethane (C2H6), methylacetylene (CH3C2H), propane (C3H8), diacetylene (C4H2), and butane (C4H8). These gases diffuse to the lower stratosphere where temperatures are low enough to initiate condensation. Particle formation may not occur readily, however, as the vapor species become supersaturated. We present a theoretical analysis of particle formation mechanisms at conditions relevant to Neptunes troposphere and stratosphere and show that hydrocarbon nucleation is very inefficient under Neptunian conditions: saturation ratios much greater than unity are required for aerosol formation by either homogeneous, heterogeneous, or ion-induced nucleation. Homogeneous nucleation will not be important for any of the hydrocarbon species considered; however, both heterogeneous and ion-induced nucleation should be possible on Neptune for most of the above species. The relative effectiveness of heterogeneous and ion-induced nucleation depends on the physical and thermodynamic properties of the particular species, the abundance of the condensable species, the temperature at which the vapor becomes supersaturated, and the number and type of condensation nuclei or ions available. Typical saturation ratios required for observable particle formation rates on Neptune range from approximately 3 for heterogeneous nucleation of methane in the upper troposphere to greater than 1000 for heterogeneous nucleation of methylacetylene, diacetylene, and butane in the lower stratosphere. Thus, methane clouds may form slightly above, and stratospheric hazes far below, their saturation levels. When used in conjunction with the results of detailed models of atmospheric photochemistry, our nucleation models place realistic constraints on the altitude levels at which we expect hydrocarbon hazes or clouds to form on Neptune.

Volcanically emitted sodium chloride as a source for Io's neutral clouds and plasma torus

The atmosphere of Jupiters satellite Io is extremely tenuous, time variable and spatially heterogeneous. Only a few molecules—SO2, SO and S2—have previously been identified as constituents of this atmosphere, and possible sources include frost sublimation, surface sputtering and active volcanism. Io has been known for almost 30 years to be surrounded by a cloud of Na, which requires an as yet unidentified atmospheric source of sodium. Sodium chloride has been recently proposed as an important atmospheric constituent, based on the detection of chlorine in Ios plasma torus and models of Ios volcanic gases . Here we report the detection of NaCl in Ios atmosphere; it constitutes only ∼0.3% when averaged over the entire disk, but is probably restricted to smaller regions than SO2 because of its rapid photolysis and surface condensation. Although the inferred abundance of NaCl in volcanic gases is lower than predicted, those volcanic emissions provide an important source of Na and Cl in Ios neutral clouds and plasma torus.

Meteoroid ablation in Neptune's atmosphere

Abstract Meteoroid ablation rates in Neptunes atmosphere are calculated for certain assumptions concerning the mass and velocity distribution of meteoroids in the outer Solar System. Silicate meteoroids lose most of their mass in the 1- to 100-μbar region of Neptunes atmosphere while water-ice meteoroids ablate at even higher altitudes, i.e., pressures ⪅1 μ bar. Although the ablation of both silicate and water ice occurs at altitudes that are too low to significantly affect ionospheric chemistry, meteors might contribute to ionization in Neptunes lower “bottomside” ionosphere. In addition, the interaction of water dissociation products with neutral atmospheric constituents can generate CO molecules in Neptunes upper atmosphere. The total column influx of water molecules due to meteoroid ablation on Neptune is found to range from 7 × 10 5 to 2 × 10 8 cm −2 sec −1 (an amount that could lead to stratospheric column abundances of CO of 5 × 10 15 to 1× 10 18 molecules cm −2 ); the exact value depends on the characteristics of the meteoroid population. Even the upper value of this range appears to be too small to explain the recent observations of CO on Neptune; however, given the uncertainty is these calculations, meteoroid ablation cannot be ruled out as the source of CO on Neptune. The total column influx of silicate material ranges from 2 × 10 6 to 3 × 10 8 cm −2 sec −1 . Even the lower value of this range is sufficient for providing a source of dust particles to initiate nucleation and condensation of hydrocarbons in Neptunes lower stratosphere.

Exoplanetary Atmospheres—Chemistry, Formation Conditions, and Habitability

Characterizing the atmospheres of extrasolar planets is the new frontier in exoplanetary science. The last two decades of exoplanet discoveries have revealed that exoplanets are very common and extremely diverse in their orbital and bulk properties. We now enter a new era as we begin to investigate the chemical diversity of exoplanets, their atmospheric and interior processes, and their formation conditions. Recent developments in the field have led to unprecedented advancements in our understanding of atmospheric chemistry of exoplanets and the implications for their formation conditions. We review these developments in the present work. We review in detail the theory of atmospheric chemistry in all classes of exoplanets discovered to date, from highly irradiated gas giants, ice giants, and super-Earths, to directly imaged giant planets at large orbital separations. We then review the observational detections of chemical species in exoplanetary atmospheres of these various types using different methods, including transit spectroscopy, Doppler spectroscopy, and direct imaging. In addition to chemical detections, we discuss the advances in determining chemical abundances in these atmospheres and how such abundances are being used to constrain exoplanetary formation conditions and migration mechanisms. Finally, we review recent theoretical work on the atmospheres of habitable exoplanets, followed by a discussion of future outlook of the field.

Phase transformations and the spectral reflectance of solid sulfur: Can metastable sulfur allotropes exist on Io?

Abstract The spectral reflectance of elemental sulfur that has solidified from a melt changes with time after the sulfur has solidified. This temporal variation arises as a result of phase transformations occuring within the solid. In a set of laboratory investigations, we find that variations in the thermal history of the sulfur samples profoundly affect the solid-state transformation rate and the corresponding spectral variation of freshly frozen sulfur. In particular, samples that were heated to 393 and 453 K for various lengths of time (up to 50 hr) and then solidified and aged at various temperatures (from 260 to 318 K) brighten at visible wavelengths on very different time scales. This temporal variation is thought to be due to differences in the amount and type of metastable allotropes present in the sulfur after solidification as well as to the physics of the phase transformation process itself. Our laboratory data have implications for the spectral variation and physical behavior of freshly solidified sulfur, if any exists, on Jupiters satellite Io. Depending on its thermal history, molten sulfur on Io will initially solidify into a glassy solid or a monoclinic crystalline lattice; these forms may contain polymeric molecules as well as the more abundant S 8 molecules. If freshly frozen sulfur on Io can lose heat rapidly and approach ambient dayside Io temperatures within a few hours, then our laboratory results imply that the metastable monoclinic or polymeric allotropes can be maintained on Io and will take years to convert to the stable orthorhombic crystalline form. We present cooling rate calculations that indicate that metastable allotropes can be preserved in small droplets of sulfur ejected during volcanic plume eruptions on Io. However, sulfur in large lakes or flows on Io might remain warm long enough for the conversion of monoclinic sulfur into orthorhombic sulfur to proceed, and we would expect rapid brightening (on the order of hours or days) in these areas after the liquid sulfur has solidified.

Upper Atmosphere and Ionosphere of Saturn

This chapter summarizes our current understanding of the upper atmosphere and ionosphere of Saturn. We summarize the available observations and the various relevant models associated with these regions. We describe what is currently known, outline any controversies and indicate how future observations can help in advancing our understanding of the various controlling physical and chemical processes.

Post‐SL9 sulfur photochemistry on Jupiter

We have modeled the photochemical evolution of the sulfur-containing species that were observed in Jupiters stratosphere after the SL9 impacts. We find that most of the sulfur is converted to S8 in the first few days. Other important sulfur reservoirs are CS, whose abundance increases markedly with time, and possibly H2CS, HNCS, and NS, whose abundances depend on kinetic reaction rates that are unknown at the present. We discuss the temporal variation of the major sulfur compounds, make abundance and compositional predictions useful for comparisons with observations, and discuss the possible condensation of sulfur-containing species.